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Cupola bed key to quality melt.

The importance of proper cupola bed burn-in is essential to good cupola melting. If the prepared bed has not been ignited fully and reached a uniform temperature level, the operation of the cupola during and immediately after tap out can be extremely erratic.

This can result in low iron temperature and subsequent flow rate, inadequate carbon pick-up and excessive silicon loss. The initial iron may be so cold (below 2450F) that it will freeze in the tap hole, under the slag skimmer or in the launder. It also can present serious safety and economic concerns involving:

* extreme physical demands on melting personnel;

* heavy reliance on oxygen lancing;

* the danger of iron overflowing the cupola well;

* intermittent operation.

Even when such operational difficulties are overcome, the associated production delays, off-analysis and cold iron situations inflict enormous nonrecoverable economic losses. Proper attention to coke bed preparation can circumvent these costly situations.

The correct bed height in the cupola is determined by physical measurement followed by an adjusting coke addition, if required, prior to adding the cupola metal charge. Bed height alone, however, is not the only factor that needs to be monitored to achieve optimum tap out. A completely ignited coke bed able to reach the ideal melt temperature is the other essential ingredient in the equation for optimal cupola production.

Bed Monitoring

The specific procedure for monitoring coke bed temperature during burn in varies from foundry to foundry, but the same basic concepts remain. A chart recorder with a 0-2500F scale connected to a thermocouple generally is adequate for most cupolas with or without hot blast or oxygen enrichment. A Type K (chromel-alumel alloy) thermocouple is suggested. Backup units with a thermocouple simulator or similar calibrating device should be available to evaluate periodically the performance of measuring equipment.

The maximum steady state temperature shown by the recorder (the temperature at which the strip chart recorder shows little or no change with time) will typically be in the range of 1700F-2500F and should be reproducible from day to day.

The temperature reached will depend upon specific conditions such as the use of hot blast and/or oxygen and the height of the free space between the top of the bed and the charge door. The recorder should be located near the cupola where the operator can observe it and make appropriate operating adjustments.

One obvious thermocouple location also is in the vicinity of the charge door. One procedure places a thermocouple five ft below the charge door sill, well away from cold outside air. Obviously, the thermocouple must be removed from the cupola when charging additional coke.

Once steady state conditions have been achieved, indicating that the bed has reached its optimum condition, the bed height should be measured. If it is above the established range, additional burn-in time will reduce it. If the bed height is low, additional coke should be added to achieve the correct height.

When the "green cap" of coke is less than about 10% of the total coke bed weight, it need not be burned again and charging can begin immediately.

Coke additions of more than 10% require the bed to be reburned until steady state conditions are again shown on the thermocouple.

In either case, another height measurement should be made before charging and after an addition and/or reburn is completed.
COPYRIGHT 1993 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Heintz, Ricklin L.
Publication:Modern Casting
Date:Jan 1, 1993
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